1. Field of the Invention
The present invention relates to a transport/storage cask for the radioactive material such as spent nuclear fuel.
2. Description of the Related Art
As the above type of technology, U.S. Pat. No. 5,641,970 discloses a transport/storage cask for a radioactive material in which a gamma ray shielding layer and a neutron shielding layer are provided between an inner shell and an outer shell. The gamma ray shielding layer is formed by a plurality of divided block bodies in the circumferential direction, and the block bodies are composed of lead.
Certainly, as mentioned in the above Patent Document, when a shielding performance with regard to gamma ray and cost are taken into consideration, lead is the most suitable for a material of the block bodies. However, as already known, since lead is easily deformed by external force, there is a need for improvement in terms of strength. Particularly, at the time of a so-called 9 m drop test, by inertia force due to impact acceleration, the block bodies of lead are locally crushed so as to extend in the horizontal direction. Therefore, there is a possibility that length in the axial direction of the transport-storage cask is shortened so as to generate a partial clearance.